1. Field of Invention
This invention relates to a waveform measuring device for making the waveform of an input signal observable; and more particularly, to an improvement of a trigger signal generating circuit that makes waveform observation easy.
2. Description of Prior Art
One type of waveform measuring device is called a digital oscilloscope which stores an input signal in a memory after converting the input signal into a digital signal, and displays a waveform on a display unit, such as a cathode ray tube (CRT), on the basis of the digital data held in the memory.
FIG. 1 is a block diagram showing an example of such a conventional waveform measuring device. In FIG. 1, an input signal is applied to an Analog-to-Digital (A/D) converter 1, which converts the input signal into a digital signal in synchronization with a pulse signal applied from a clock pulse generator 8, the digital signal then being stored in a memory 2. The input signal is also applied to a trigger circuit 4. When the input signal reaches a given level which is preset, trigger circuit 4 applies a signal to a trigger control circuit 5. Trigger control circuit 5 applies a control signal to memory 2 and to a display control circuit 6 on the basis of the output from trigger circuit 4. Display control circuit 6 applies a control signal to memory 2 and to a sweep generating circuit 7 on the basis of an output signal from trigger control circuit 5 and a clock pulse signal from clock pulse generator 8. The data held in memory 2 is applied through a Digital-to-Analog (D/A) converter, not shown, to the Y-axis of CRT3, whereas the output signal from sweep generating circuit 7 is applied to the X-axis, whereby a waveform is displayed on CRT 3. During waveform displaying, to make the observation easier, the moment when the input signal reaches a given preset level is made to accord always with the left end of a display area of CRT 3. An unblanking circuit 9 is provided for inhibiting unnecessary displaying.
The operation of the conventional waveform measuring device will now be described in connection with its use to display an input signal whose waveform varies irregularly, as shown in FIG. 2(A). The dotted line in FIG. 2(A) represents a trigger level TL. When the input signal crosses the trigger level TL in the low-to-high direction of level, i.e. at moments t.sub.1, t.sub.2, t.sub.3, a trigger signal is generated. As a result, display of the waveform on a CRT 3 takes place in a manner such that the points of these moments t.sub.1, t.sub.2, t.sub.3 always accord with the left end of the CRT.
However, since individual portions of the waveform of FIG. 2(A) subsequent to moments t.sub.1, t.sub.2, t.sub.3 differ from one another, these portions are displayed in superposed form as shown in FIG. 2(B), thereby making waveform observation impossible.
To overcome the defects of such a conventional device, one of the present inventors has invented an improved digital oscilloscope which is disclosed in Japanese Patent Laid-Open No. 63/298,166. This digital oscilloscope is such that an input signal is converted into a digital signal by an A/D converter and then stored in a memory. Past data which occurred at a certain time backward from the present moment in time is read out from the memory. Only when the present input signal crosses the trigger level while the past data is within a preset range, is a trigger signal generated and a waveform displayed. By such a device, it is possible to display a specified portion alone of a complicated irregular waveform, thereby making waveform observation easy and reliable.
However, disadvantageously, the past data must be read out of the memory and compared with the preset range for the purpose of generating the trigger signal. Thus, not all of the data can be utilized which correspond to past moments remote from the present moment. Only data requiring the shortest time necessary to write data in the memory and to read data out of the memory can be utilized.
Conceptually a system could be conceived which operates in such a manner that among the data held in a memory, a plurality of data blocks having a desired temporal relationship therebetween are read out, and each data block is compared with a preset level by software processing to generate a signal identical to the trigger signal. Such a system, unfortunately would need a long time for software processing. Thus, such a system could not be utilized for reat-time measurement.